Method of producing a humidity sensor by shadow casting and resultant product



, D. A. KoHL 2,860,221 METHOD OF P O UC A HUM I S SOR BY NG RESULT ed Nov. 25, 1

Nov. ll, 1958 SHADOW R D CASTI Fil INVENTOR. OUGLS A O/l..

BY I M2M United States Patent O METHOD F PRODUCING A HUMIDITY SENSOR BY SHADOW CASTING AND RESULTANT PRODUCT Douglas A. Kohl, Osseo, Minn., assigner to `General Mills, Inc., a corporation of Delaware Application November 25, 1955, Serial No. 548,977 7 Claims. (Cl. 20163) This invention relates generally to humidity sensing sensor with which the present invention is concerned, it

may be explained that the term adsorbing is herein defined, in contradistinction to absorbing as encompassing or involving a characteristic in which there is a preferential attraction for water vapor molecules and while said molecules are in contact with the adsorber there is no chemical change or irreversible action. Absorption, on the other hand, occurs when water vapor molecules are actually combined through chemical reaction with the absorber, incorporated into the molecular structure of the absorber (for instance, water of crystallization), or there is a release of ions by virtue of a binding action such that the conduction of electric current by the ions may result in physical changes. Typical of such reactions are the swelling of cellulose and the ion-exchange resins.

Where adsorbing materials such as quartz, fused quartz and high silica glass are employed in the construction of humidity sensing elements, it frequently happens that the resistance range is too high for practical purposes. Accordingly, one object of the invention is. to provide a humidity sensor and a method of making such sensor that will lower" the resistance range of humidity sensors functioning on the aforementioned adsorbing principle to such an extent that the humidity element is more useful. One nicety of the invention resides in the fact that the exposed surface area of'a humidity sensing element manufactured in accordance with the teachings of the instant invention is decreased without decreasing the probability of adsorption. More specifically, the invention contemplates the steps of rst roughening the surface to be used, such as by grinding or etching, and then lshadow casting discrete metallic areas onto portions of the minute peaks or hills, leaving the crevicesV or valleys uncoated. Since itis in the crevices that most of the moisture will accumulate, it will be recognized that by use of shadow casting techniques the metal is applied to regions of relatively poor adsorption, these regions contributing comparatively little to the functioning of the sensor. Stated somewhat differently, the applied small areas of metal serve to bridge at least partially the regions where a lower conductivity is experienced by virtue of a lesser thickness of moisture film being present, that is where the high spots exist.

Another object of the invention is to provide a method lending itself readily to the controlling of the resistance range during fabricatio-n so that it will fall within desired limits.

Other objects will be in part obvious, and in part pointed out more in detail hereinafter. K

The invention accordingly consists in the features of construction, combination of elements and arrangement .of parts which will be exemplified in the construction hereafter set forth and the scope of the application which Will be indicated in the appended claims.

In the drawing:

Figure l is a perspective view, partly broken away, showing a suitable apparatus for carrying out the improved method or process embodying my invention;

Fig. 2 is an enlarged sectional view showing the relative disposition of the adsorber plate and the evaporating boat, the view being before rotation of the adsorber plate, and

Fig. 3 is a view similar to Fig. 2, but showing the adsorber plate after it has been rotated through 180.

Referring in detail to the drawing, certain apparatus appurtenant to the method by which the invention may be'practiced is illustrated in Fig. 1. Such suitable apparatus suggestvely includes a supporting base upon which is mounted a housing 12. The housing 12 may be in the form of a bell-jar or the like having a closed dome-shaped or semispherical top and a bottom open end having an annular flange 13 which is adapted to rest upon the top face of the supporting base 10 in a gas tight manner. For the purpose of evacuating air from the housing 12, there is provided a nipple or opening 14 to which an exhaust pump (not shown) is attached.

Within the housing 12 is a stand 16 comprising an upright 18 upon which is mounted a vertically slidable collar 20, there being a set screw 22 by which the collar can be retained at a preferentially adjusted elevation. Integral with the collar so as to be movable therewith is a sleeve bearing 24 having a rotatable shaft 26 journaled therein. To one end of the shaft 26 is affixed a pulley 28 and trained over this pulley is a belt or cord 30 which is driven by another pulley 32 on the shaft of a small electric motor 34.' Suitable electrical connections for the motor extend through the base 10 but these are not herein pictured.

At the end of the shaft 26 opposite from the pulley 28 is a clamping device composed of a strip of spring metal forming a pair of gripping fingers 36. These gripping fingers 36 may be manually spread apart sufficiently for the accommodation therebetween of a plate 38, such as glass, having previously applied interleaved grids 40 and 42 on one face thereof. These grids may be formed by a metal evaporation process somewhat analogous to the one utilized in the realization of the instant invention, employing noble metal such as gold or platinum and a mask through which the evaporated metal passes.

However, prior to the application of the grids 40 and 42 the face 44 is roughened, either by etching or fine grinding, The etching procedure may be performed by exposing the face 44 to uoboric acid fumes and the grinding may be accomplished by using #500 or finer grit, the operation being conducted in a random fashion.

With respect to the facial configuration produced by the above-described roughening action, whether by etching or grinding (including sand blasting), attention is directed to Fig. 2 where a greatly magnified cross-section of the glass plate 38 is presented. From this figure it can be perceived that the contour of the surface is highly irregular, being formed of numerous peaks 46 and valleys or crevices 48. Because of this uneven surface outline, it will be observed that the actual distance between grids along the irregular profile of the face 44 is much greater than any straight line distance would be. Also, the moisture layer that will form on the face 44 will tend to form pools in the valleys 48, leaving a thinner layer of moisture on the various intervening peaks 46.

It is with the peaks 46 and their lower electrical conductivity that we are currently concerned, and it is within the purview of the invention to coat these peaks, or portions thereof, with metal so as to lower the over-all resistance between grids. To this end, a boat or crucible the grids 40, 212 are` toA be applied before or `after A50 is suspended between two vertical posts 52, suitable electrical connections (not visible) passing upwardly through these posts so as to heat the boat 50 resistively. Other modes of heating, ofcourse, maybe employed.

At any rateit is-to Vbe noted Athatwthe boat r50' is located slightly forward of the plate `38 anda considerabledistance therebelow. A. -As best seen in'v Fig. 2 the boat formsan acteangle of aboutrlQr" withr'espect to the plate '38. Consequently,)1`nany of the peaks 46 provide a shielding action for the valleysr48 on theefar side of the peaks. y f

ln order to cause formation of discrete metallic areas, which we will collectively represent by the numeral 54am Fig.` 2, a jnoble metal `56 such as gold or platinum is "placed -in wire or powder form in the boat '50.' When the boat is heated Vthis metalr56 will become molten and emit minute :particles otfmolecular size which will4v emanate in straight lines in diierentdirections withinfthe housing l2. 'Those straight paths with' whichrwe are interested are those heading Y-tvvard the glass plate 3.8. Accordingly, tliese particular paths` have been'designated lin phantom'outline and Ybearthe reference 'numeralSft V1t V1s these' dotted linesthat produce thepreviously'alluded to angle of 10 which of coursecanrbe varie-das particular circumstant'z'es dictate.

In Fig.` 2V we Vwill assumethat no rotation ofthe shaft 26'has taken place and that the plate 38 is 'therefore in vits initial. position. Hence, the areas or -islands V54 are all on the underside of the peaks 46. However, the invention in'eludes the VV'idea of controlling Athe amountof metal to be. deposited onto the peaks, yand Ywith. this in mind structure 16 has been setforth in Fig.v l, as already described, for turning the plate 38 in a. spit-like manner.

and receive evaporated metal. Y

`By governing thedegree Vof angular turning ofthe vplate 38 by the simple expedient `of stopping themotor '34 after it'has rotated through a desired angle, the overall area receiving metal is controlled. Owing' to the acute angle (10 in the exemplified situation) thevalleys '48 do not receive metal and these valley-regions'there- Vfore do not havel their conductivity increased as-do"` the peaks. v v Y e After fabrication and when the humidity sensor'is .subjectedto a humidvatmosphere, electrical energization ofthe grids n$0,742 will permit a humidity measurement yfto be' made. n By virtueof the presence oftheV metallic Vareas 54, cupled with those areas 60 derived front-whatever rotation has been made, the resistance characteristics are modiedso thatthe over-all resistance range for different humidity states will! be lowered..fromwhat would otherwise occur. As hereinbefore stated, the'pro- .posed surface alteration resultsfin a more optimum operating procedure than'if no treatment is :undertaken It mightV be mentioned that inlmanufacturing humidity sensing lelements in accordance with thef invention,-it will be left up to the designer to decide'as Vto Whether the shadow. casting step.

As many changes could be made in the above construction andmany apparently widely different ernlodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall he Vinterpreted as illustrative and not in a limiting sense. T

lt is also to vbe understood that thelanguage yused in the following claims is intended V'to cover allofthe generic and specific features of the invention'hereindescribed and all statements of the scope of the invention which, as a matter of language, might 'be said to fall therebetween.

What is claimed: Y

l. A humidity sensor comprising an inert base having a roughened surface of high electrical resistivity composed of peaks and valleys, relatively small metal areas on the top portions only of the peaks butfnot the valleys, and interleaved metallic gridsspaced from each other -on said surface.

2. A humidity sensor comprising an adsorbing ele- .ment having a roughened surface of high electrical resistivity thereon composed of a multiplicity of.v peaks and valleys, discrete metallic Vareas on the top portions onlyof said peaks, and interleaved metallic grids spaced somewhatfrom each other on said surface. Y

3. AA humidity sensor in accordance with claim'2in which saidmetallic areas all facein a single'direction.

4. Ai humidity sensor in accordance with claim 2 in ywhichnthe'metallic areas face in thersame general direction within an acute angle subtended by a preferred arc.

4'5."AA method of making a humidity sensor comprisingthefstep of'subjecting an adsorbing element having av roughened'surface of high electricalresistivity 'composed of peaks andrvalleys to evaporated metal, the 'plane of the roughened surface being disposed `at a'nacute angle to'the directionof impingementuof said metal'so as to coatthe top portions only of said peaks butV not the valleys. Y d, ,i

V6. A method in accordance with claim 5 in whichjsaid 'acuten angle -is approximately 10".l j,

7. Al method of Vmaking a humiditysensor comprising the steps uof disposing an adsorbing element having a roughened surface of high electrical resistivity. cornpose'd of peaks and valleys to a source offevaporated metal, the plane. ofthe roughened surface beingfatf an acute anglerelativetothe direction of impingement'of said metal so as to exposefthe top., portions only ovfsaid peaks -to said-source, and rotatingsaid'element whilefat i-'saidpr'eferred*angle torcoat thereby various top portions of 'said peaks with evaporated metal.

References cited rintheaibfniis patent -UNITED Stearns? PATENTS 

